In FY '84 we reported that the binding constant for AMP-PNP and PPi binding to attached crossbridges in fibers was considerably weaker than that reported for binding to acto-S1 in solution. New biochemical evidence, however, suggests that the strength of binding of AMP-PNP and PPi to acto-Sl is similar to that obtained in our fiber experiments. To further explore the relationship between the fiber and solution binding constants for nucleotide and analogue, in FY '86 we measured the binding of ADP to attached crossbridges. We found it to be moderately strong, about as strong as the binding of ADP to acto-S1 in solution. On balance, therefore, the evidence suggests the binding of nucleotides and the nucleotide analogues to crossbridges is about as strong as that to acto-S1 in solution. We have also measured the crossbridge detachment rate-constants in the fiber and compared them with those for detachment of S1 from actin in solution. We detect crossbridge detachment by measuring the rate of decay of force after rapid stretch of a single isolated muscle fiber. This is possible since the decay of force after stretch is due to strained crossbridges detaching and then reattaching in positions of lesser strain. We find that in the fiber the decay of force following a stretch is not described by a single rate-constant but by multiple rate-constants spanning a wide range. The fastest rate-constants are comparable to the detachment rate-constant of S1 from actin in solution but the majority are much slower. In FY '86 we ruled out the possibility that this slowness in apparent detachment rate was related to crossbridge crowding. Currently, experiments are underway exploring other possible explanations.